Travelling wave tubes



H. A. c. HOGG 2,973,453

TRAVELLING WAVE TUBES Feb. 28, 1961 Filed April 22. 1959 2 Sheets-Sheet 1 INVENTOK ,Feb. 28, 1961 H. A, c. HOGG 2,973,453

TRAVELLING WAVE TUBES Filed April 22, 1959 2 Sheets-Sheet 2 ug Aim 1 Unite TRAVELLING WAVE TUBES Harold Alexander Cecil Hogg, Frinton-on-Sea, England,

assignor to The l -O Valve Company Limited, Hammersmith, London, England Filed Apr. 22, 1959, Ser. No. 808,206

Claims priority, application Great Britain Apr. 24, 1958 Claims. (Cl. 315-35) This invention relates to travelling wave tubes.

In a travelling wave tube, an electron beam is arranged to travel along a channel included in an evacuated enclosure and to interact with an electromagnetic wave arranged to travel along a delay device which at least partly defines said channel. In general, the delay device is at least partly in the form of a periodic metal structure; by a periodic metal structure is meant a series of similar metallic elements which are spaced apart along a given path (the length of the structure) and all of which extend in a similar manner transversely to said path. The elements may for example take the form of the turns of an elongated helix, the teeth of one or more comb-like structures or the rungs of a ladder-like structure. In order to achieve maximum interaction between the electron beam and the electromagnetic wave, the beam should pass as close as possible to the gaps in the periodic structure forming the delay device since the axial electric field of the electromagnetic wave is at a maximum in the regions of these gaps. This requirement is especially important in backward wave 0scillators since the space-harmonics of the field of the electromagnetic wave decay more rapidly away from the structure than the fundamental component of the field, and, in such a device, the electron beam is arranged to interact with a reverse space-harmonic of the field; by a reverse space-harmonic is meant a space-harmonic whose phase and group velocities are directed in opposite senses. The achievement of such maximum interaction however involves certain difficulties. For example, when the electron beam travels in a rectilinear direction, the gaps in the periodic structure are shielded from the electron beam by the elements between which the gaps are formed. Such shielding may be reduced by making each element very thin in a direction perpendicular to the direction of travel of the electron beam, but in such a case the periodic structure will be relatively mechanically weak and will have a low thermal capacity so that distortion or even melting of the ele ments may occur in operation.

A further requirement of a travelling wave tube is that some form of focussing arrangement must be provided in order to prevent the electron beam from diverging unduly as it travels along the channel; for example, in the case of a travelling wave tube employing a helix as a delay device, such focussing is commonly achieved by means of electromagnets arranged so that there is a strong magnetic field directed along the axis of the helix.

It is accordingly an object of the present invention to provide a travelling wave tube in which strong interaction is achieved in operation between the electron beam and an electromagnetic wave arranged to travel along a delay device incorporated in the tube and in which the difiiculties discussed above are alleviated.

It is a further object of the present invention to provide a travelling wave tube which includes convenient means for focussing an electron beam.

. According to the present invention, a travelling wave tube includes a delay device which is at least partly in the form of a periodic metal structure, two further metal structures extending on either side of the periodic metal structure, a first one of the further metal structures being disposed appreciably nearer to the periodic metal structure than the other further metal structure, and an electron gun having an electron source, the gun being disposed adjacent one end of the periodic metal structure and being arranged to project an electron beam in a suitable manner into the space between the periodic structure and said other further metal structure, the arrangement being such that, in operation, upon applying suitable potentials to the periodic structure and to the further metal structures such that the periodic structure is at a positive poential with respect to the electron source, said other further metal structure is at a negative potential with respect to he periodic structure and said first further metal structure is at a negative potential with respect to said other further metal structure, the elecron beam will travel through said space generally in a direction parallel to the length of the periodic structure and with an undulatory motion such that it alternately approaches and recedes from the periodic structure with those peaks of the beam forming that set of alternate peaks disposed adjacent the periodic structure being respectively disposed in the vicinities of the gaps in the periodic structure.

One arrangement in accordance with the invention will now be described by way of example with reference to the accompanying drawings, in which:

Figure 1 is a side elevation, shown partly in section, of a travelling wave tube adapted to operate as a backward wave oscillator, the section being taken along the longitudinal axis of the tube;

Figure 2 is an enlarged sectional elevation of the tube, the section being taken along the line II-II of Figure 1;

Figure 3 is a perspective view of part of a periodic metal structure incorporated in the tube; and

Figure 4 is a schematic representation of part of the tube, illustrating its mode of operation.

Referring to the drawings, the delay device of the of rectangular cross-section, which is formed by two travelling wave tube includes a tubular metal member 1, channel members 2 and 3 of U-shaped cross-section. The delay device also includes a periodic metal structure 4 which extends along the whole length of the member 1, and two metal focussing plates 5 and 6 which are disposed inside the member 1 and on either side of the periodic structure 4; the plates 5 and 6 extend for almost the whole length of the member 1 with their main faces parallel to the bases of channel members 2 and 3.

The focussing plates 5 and 6 each have a width of 0.5 centimetre, and are accurately positioned in the member 1 with their adjacent main faces spaced 0.12 centimetre apart, the plates 5 and 6 being respectively secured to the bases of the channel members 2 and 3 by means of a number of studs 7 one end of each of which is brazed to the relevant plate 5 or 6. Each plate 5 or 6 is accurately spaced, and electrically insulated, from the relevant channel member 2 or 3 by means of a number of ceramic washers 8, and each channel member 2 or 3 is electrically insulated from the heads of the relevant studs 7 by means of a number of further ceramic washers 9; each washer 9 is held in position by means of a spring clip 10 which is trapped between the washer 9 and the head of the associated stud 7. It should be appreciated that the plates 5 and 6 effectively form the ridges of a ridged waveguide the walls of which are formed by the tubular member 1.

Referring now particularly to Figure 3 of the drawings, the periodic structure 4 includes a series of seventy metal strips 11, each 0.038 centimetre wide except for the end strip at one end of the structure, which extend transversely to the length of the periodic structure 4, the ends Patented Feb. 28, 1961 of each strip 11 being respectively formed integral with two longitudinal strips 12 and 13. The strips 11 are spaced equal distances apart, a slot 14, 0.089 centimetre wide, being formed between each adjacent pair of strips 11. The strips 11 are all bent in a simi'larmanner so that each strip 11 is formed by three straight portions, and the periodic structure 4 is mounted in position in the member 1 with the longitudinal strips 12 and 13 clamped between the channel members 2 and 3 (see Figure 2), the periodic structure 4 being so arranged that the strips 11 each present a concave configuration to the plate 6. The periodic structure 4 is arranged so that the central straight portion of each strip 11 extends across the whole width of the plates 5 and 6 and is disposed parallel to the main faces of the plates 5 and 6, the central portion being disposed 0.018 centimetre from the plate 5 and 0.084 centimetre from the plate 6.

A number of metal pins are secured to the long narrow sides of the plate 5, the pins 15 projecting just over half-way across the gap between the plates 5 and 6. The arrangement is such that a pair of pins 15 project through each slot 14, each pair of pins 15 being symmetrically disposed with respect to the relevant slot 14. For convenience, each series of pins 15 secured to a side of the plate 5 is formed from a periodic structure similar to the structure 4 except that it is flat, the pins 15 being formed by parts of the transverse strips of the structure and being connected together by one of the longitudinal strips of the structure.

The travelling wave tube includes a collector electrode 16 part of which is disposed transverse to the length of the tube and about 0.25 centimetre from one end of the periodic structure 4, the electrode 16 being secured to the channel member 3 by means of two bolts 17 (only one of which is seen in the drawings) the electrode 16 is electrically insulated from the channel member 3 by further ceramic washers 18. p

The travelling wave tube includes an electron gun-19 disposed adjacent that end of the tubular member 1' remote from the collector electrode 16 and designed to project a strip-like beam 29 of electrons into the space between the periodic structure .4 and the plate 6. The electron gun19 includes an indirectly heated cathode 21, three anodes 22, 23 and 24, and a heat shield 25. The anode 24 is in the form of a plate which closes that end of the tubular member 1 remote from the collector electrode 16, and the anodes 22, 23 and 24 respectively have formed in them three strip-like apertures such as 26 which extend in directions parallel to the main faces of the plates 5 and 6 and through which the electron beam passes before emerging from the gun 19 in operation; the aperture 26 has a width of 0.063 centimetre.

As is clearly shown in Figure 4 of the drawings, that strip 11 of the periodic structure 4 adjacent the electron gun 19 is wider than the remaining strips 11 and abuts against the anode plate 24 on that side of the aperture nearer the plate 5.

The assembly described above is mounted in an evacuated tubular metal envelope 27 of circular cross-section, the majorpart of which has an external diameter of 1.9 centimetre; A number of pins 23 are sealed through an insulating base 29 at one end of the envelope 2'? and are respectivelyelectrically connected to the focussing plates 5 and 6, the collectorelectrode 16, the cathode 21, and the anodes 22 and 23 by means of schematically indicated electrical leads 30. A pumping stem 31 is provided at the other end of the envelop e2'7, the stem 31 being pinchedofi when the evacuation of the envelope 27 has been completed duringmanufacture. The tubular member 1 is supported inside the envelope 27 by virtue of the longitudinal edges of the memberl bearing against the inner surface of the envelope 27 as clearly shown in Figure 2.

A tubular metal memberSZ'; of circular cross-section end of the member 1 adjacent to the electron gun 1%, the axis of the member 32 being perpendicular to the axis of the envelope 27 and one end of the member 32 being secured to the base of the channel member 3. A metal rod 33 passes through, and is arranged coaxially with, the tubular member 32; one end of the rod 33 is brazed to the focussing plate 6, the rod 33 passing through a circular aperture 34 formed in the channel member 3 in register with the narrow end of the bore of the member 32. The free end of the rod 32 is surrounded by a glass bulb 35 sealed to a circumferential flange 36 provided at the free end of the member 32 so as to hermetically seal the envelope 27. A mica washer 37 fits tightly around the rod 38 and inside the member 32, the washer 37 serving to locate and support the rod 33 with respect to the member 32. The end of the rod 33 is designed to project into, and approximately half-Way across the narrow internal dimension of, a waveguide 38 which is provided with an aperture to accommodate the member 32. The tubular member 32 and the rod 33 together constitute a section of coaxial conductor transmission line which forms a radio frequency output con nection for the travelling Wave tube, one end of the line being effectively connected to one end of the delay device constituted by the tubular member 1, the periodic structure and the platesS and 6, and the other being terminated by an antenna which is formed by the projecting end of the rod 33 and is adapted to excite a wave in the Waveguide 38.

in operation, the cathode 21, the anodes 22, 23, and 24, and the focussing plates 5 and 6 are connected via the pins 25 to appropriate potential sources (such for example. as the potential sources A, B and C) such that the anode 24 (which is electrically connected to the periodic structure 4, the tubular member I. velope 2'7) is carthed, an appropriate negative potential is applied to the cathode 21, a potential of about 12 volts positive with respect to the cathode potential is applied to the anode 22, a negative potential of about volts is applied to the .anode23, an appropriate negative potential is applied to the plate 5 and an appropriate egative potential (less than that applied to the plate 5) is applied to the plate 6; for example, those pins 23 which are respectively electrically connected internally to the periodic structure t and the plates 5 and 6 are respectively electrically connected externally to the potential sources A, B and C. The potential applied to the cathode 21 can be varied between 300 and 1500 volts, the potential applied to the plate 5 can also be varied between 300 and 1500 volts and the potential applied to the plate 6 can be varied between 100 and 500 volts; the ratios of the potentials respectively applied to the cathode 21 and the plates 5 and 6 should be kept substantially constant.

The arrangement is such that the electron beam 2% is focussed as it travels along the channel partly defined by the periodic structure 4 and the plate 6, the focussing action (which is illustrated in Figure 4) being brought about as follows. The application of the potentials to the periodic structure 4 and the plates 5 and 6 causes an electrostatic field to be set up between each strip 11 of the periodic structure i and each of the plates 5 and 6, the field being directed away from the strips 11 towards the plates 5 and 6.- After the electron beam 29 emerges from the aperture 26, it comes under the influence of the electrostaticfield between the structure a and the plate 5 and is directed towards the first slot 14 in the structure 4. Upon reaching the region of this slot 14, the beam 20 is bent away from the plate 5 and towards the plate 6 due to theelectrostatic field between the plates 5 and 6 passing through this slot 14. The beam 20 is then bent again towards the plate 5 by virtue of the electrostatic field between the first strip 1 1 and the p a Th tsl e w ali fie d .s ist butiqn alo and the enthe general direction of travel of the beam 20 is such that there is an overall focusing action upon the beam in di rections perpendicular to the plates 5 and 6 and the beam 28 travels along the channel defined by the structure 4 and the plate 6 with an undulatory motion such that it approaches the plates 55 and 6 in turn until it is incident upon the collector electrode 16, one set of peaks of the beam 28 being respectively disposed in the vicinities of the slots 14.

in operation, oscillations are set up in the travelling wave tube by virtue of interaction between the electron beam 29 and an electromagnetic wave propagated along the delay device constituted by the tubular member 1, the periodic structure 4 and. the plates 5 and 6, in a sense opposite to the direction of flow of the electron beam 26. A radio frequency output signal is obtained from the travelling wave tube by means of the wave guide 38 and the section of coaxial conductor transmission line formed by the tubular member 32 and the rod 33. The oscillation frequency of the travelling wave tube can be varied by varying the magnitude of the potentials applied to the cathode 21 and to the focussing plates 5 and 6.

it is found that, in the arrangement described above, there is no significant dispersion of the electrons in the beam 20 in directions perpendicular to the plates 5 and 6 and that a large percentage of the electrons passing through the aperture 26 are collected by the collector electrode 16. The loss in transmission that does occur is mainly due to lateral dispersion of the beam 20 in directions substantially parallel to the plates 5 and 6; it should be appreciated that such dispersion is considerably reduced by virtue of the provision of the pins 15.

As stated above, the peaks forming one set of alternate peaks of the beam 2%) are respectively disposed in the vicinities of the slots 14 formed in the periodic structure 4, and it should be appreciated that each of these peaks coincides with a region of maximum axial electric field so that in operation there is very good interaction between the electron beam 26 and the electromagnetic wave.

It should be understood that in an alternative arrangement to that described above it may be possible for the focussing structures to be substantially in the form of two concentric circular cylinders and for the periodic structure also to be generally in the form of a circular cylinder disposed between, and concentric with, the focussing structures; the electron beam in this case would be arranged to follow a sinuous path between the periodic structure and one of the focussing structures.

In the travelling wave tube described above, the spacing between the focussing plate 5 and the periodic structure 4 is less than that between the focussing plate 6 and the periodic structure 4, and it should be appreciated that the magnitudes of the potentials which must be applied tothe focussing plates 5 and 6 in order to obtain focussing of the electron beam 20 are less than would be required if the spacing between the focussing plate 5 and the periodic structure 4 were equal to that between the focussing plate 6 and the periodic structure 4.

I claim:

1. A travelling wave tube including a delay device which is at least partly in the form of a periodic metal structure, two further metal structures respectively extending on either side of the periodic metal structure, said further metal structures being electrically insulated from each other and from the periodic metal structure, a first one of the further metal structures being disposed appreciably nearer to the periodic metal structure than the other further metal structure, and an electron gun hav ing an electron source which is electrically insulated from the periodic metal structure, the gun being disposed adjacent one end of the periodic metal structure and being arranged to project an electron beam into the space between the periodic structure and said other further metal structure so as to travel along the length of said space, the arrangement being such that, in operation, upon applying suitable potentials to the periodic structure and to the further metal structures such that the periodic structure is at a positive potential with respect to the electron source, said other further metal structure is at a negative potential with respect to the periodic structure and said first further metal structure is at a negative potential with respect to said other further metal structure, the electron beam will be focussed electrostatically, the beam travelling through said space generally in a direction parallel to the length of the periodic structure and with an undulatory motion such that it alternately approaches and recedes from the periodic structure with those peaks of the beam forming that set of alternate peaks disposed adjacent the periodic structure being respectively disposed in the vicinities of the gaps in the periodic structure.

2. A travelling wave tube according to claim 1, in which the delay device includes a tubular waveguide having the periodic metal structure disposed within it and extending along its length, and said further metal structures eifectively form a pair of ridges extending along the interior of the waveguide.

3. A travelling wave tube according to claim 1, in which that part of the surface of each of the said further metal structures adjacent the periodic metal structure is substantially planar, and the periodic metal structure is at least partly formed by a substantially planar portion disposed between, and parallel to, said planar surfaces of said further metal structures.

4. A travelling wave tube according to claim 3, including means for inhibiting lateral dispersion of the electron beam in directions parallel to said planar surfaces of said further metal structures as the beam travels through said space.

5. A travelling wave tube according to claim 4, in which said means takes the form of two series of metal pins which are electrically connected to one of said further metal structures and which extend through the gaps in the periodic structure, the two series of pins being spaced apart in a direction perpendicular to the length of the periodic structure and being so arranged that in operation the beam travels between the two series of pins.

References Cited in the file of this patent UNITED STATES PATENTS 2,680,825 Warnecke et al. June 8, 1954 2,745,983 Dohler et al May 15, 1956 2,794,935 Mourier et al. June 4, 1957 2,823,332 Fletcher Feb. 11, 1958 2,857,548 Kompfner et a1 Oct. 21, 1958 2,911,556 .Charles et a1 Nov. 3, 1959 FOREIGN PATENTS 773,709 Great Britain May 1, 1957 

